Origins, evolution, and diversification of zooplankton

Fossil plankton an difficult to identify but have formed a major component of most marine ecosystems throughout geological time. The earliest fossil h eterotrophs include planktic forms, and subsequent adaptive innovations qui ckly appeared in the plankton; these include metazoans and animals with har d parts. Movement into the plankton occurred sporadically throughout geolog ical history and seems to have been independent of any biological or enviro nmental forcing mechanism. Subsequent radiations and extinctions in the coh ort of plankton closely reflect events in the benthos. The diversity of zoo plankton rose quickly during the early Paleozoic era, but low plankton dive rsity characterized the late Paleozoic. Significant radiations during early Mesozoic times led to an overall increase in diversity through the Phanero zoic eon. As the composition of the zooplankton has changed, so has their e ffect on biogeochemical cycles.